MPG Calculator
Your real miles-per-gallon
Find your car's real-world fuel economy. Enter the miles driven on a tank and the gallons it took to refill.
How to measure MPG properly
A miles per gallon calculator is really a fuel economy calculator with one input you have to gather yourself.
The method matters more than the arithmetic, which is just miles driven ÷ gallons added.
- Fill the tank completely and note the pump click-off. Do not top up past it.
- Reset the trip meter at the pump, not later.
- Drive normally — a whole tank gives a better average than a single journey.
- Refill at the same pump if you can, to the same click-off, and record the gallons.
- Divide miles by gallons. 320 miles on 11.5 gallons is 27.8 MPG.
The single most common error is the inconsistent fill. Pumps click off at slightly different points, and topping up "just a bit more" on one fill and not the next can move the result by several MPG on a small tank. That is also why averaging three or four tanks gives a far more trustworthy figure than any single measurement.
Fuel economy and cost per mile
| Miles | Gallons | MPG | Cost per mile |
|---|---|---|---|
| 250 | 14.0 | 17.9 | $0.196 |
| 320 | 11.5 | 27.8 | $0.126 |
| 400 | 10.0 | 40.0 | $0.088 |
| 500 | 12.5 | 40.0 | $0.088 |
Why yours differs from the sticker
The EPA window-sticker figure comes from standardised laboratory test cycles, adjusted downward to better reflect reality. It is a fair basis for comparing vehicles and a poor prediction of what any individual will see.
- The dashboard readout is also an estimate. It infers consumption from injector timing rather than measuring fuel, and typically reads 2 to 5% optimistic.
- Short trips are the biggest penalty. An engine below operating temperature runs rich, so a commute of a few miles can be 20% below the rating.
- Winter costs 10 to 20% through cold starts, winter fuel blends and lower tyre pressure.
- City driving is structurally worse because every brake application throws away energy you paid for.
A falling MPG is a diagnostic signal
This is the underrated reason to track it. Fuel economy is one of the few whole-vehicle health indicators available without a scan tool, and a sustained drop of 10% or more with no change in your driving usually means something specific:
| Cause | Typical loss |
|---|---|
| Under-inflated tyres | 2 to 3% |
| Dirty air filter (older engines) | up to 10% |
| Failing oxygen sensor | up to 40% |
| Dragging brake caliper | Highly variable, often large |
| Worn spark plugs | up to 30% in severe cases |
A failing oxygen sensor is the classic case: it rarely triggers a warning light immediately but can cost a substantial share of your fuel for months. Judge against your own rolling average rather than a single tank, since one bad fill can look like a fault.
MPG outside the US
| Unit | Equivalent |
|---|---|
| US MPG | 30.0 |
| Imperial (UK) MPG | 36.0 |
| Litres per 100 km | 7.8 |
| km per litre | 12.8 |
An imperial gallon is about 20% larger than a US one, so UK figures always look better for the same car — a genuine source of confusion when comparing models across markets. Litres per 100 km is the more honest unit for comparisons, because it is linear in fuel consumed rather than in distance.
Turn your measured figure into trip costs with the fuel cost calculator.
How this mpg calculator works out its numbers
Fuel economy is miles driven ÷ gallons added. Adding a fuel price also returns cost per mile, which is price ÷ MPG.
- US gallons throughout. An imperial gallon is roughly 20% larger, so UK MPG figures read about 20% higher for the same vehicle and are not directly comparable.
- Tank-to-tank measurement assumes consistent fills. Pump click-off points vary, so a single tank can be several MPG out; average three or four for a dependable figure.
- Reflects real conditions, unlike an EPA rating derived from standardised test cycles. Your figure legitimately differs from the sticker.
- Cost per mile is fuel only and excludes depreciation, maintenance, tyres and insurance.
- Diagnostic loss figures in the guide are typical published ranges and vary considerably by vehicle age and fault severity.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
MPG Calculator: frequently asked questions
How do I calculate MPG?
Divide the miles driven since your last fill-up by the gallons it took to fill back up. 320 miles on 11.5 gallons is 27.8 MPG. Average three or four tanks for a reliable figure.
Why is my real MPG lower than the sticker?
The EPA figure comes from standardised lab cycles. Short trips, cold weather, city driving, cargo and aggressive acceleration all reduce real-world economy, and a short commute can run 20% below the rating.
Is the dashboard MPG readout accurate?
Usually optimistic by 2 to 5%. It infers consumption from injector timing rather than measuring fuel directly. Calculate it manually from miles and gallons for a true figure.
What is a good MPG?
It depends on vehicle class: compact cars commonly reach 30 to 40 MPG combined, hybrids 45 to 55, and trucks and large SUVs 18 to 22. Compare against your own model's rating rather than across classes.
Why did my MPG suddenly drop?
A sustained drop of 10% or more with unchanged driving usually has a cause: under-inflated tyres, a dirty air filter, worn spark plugs, a dragging brake caliper or a failing oxygen sensor, which alone can cost up to 40%.
How many tanks should I measure?
Three or four. Pumps click off at slightly different points, so a single tank can be several MPG out. A rolling average across several fills is far more trustworthy and makes real changes visible.
How do I convert MPG to litres per 100 km?
Divide 235.2 by your US MPG. 30 US MPG is 7.8 L/100 km. Note that imperial UK MPG is a different unit again — 30 US MPG is 36 imperial MPG, because a UK gallon is about 20% larger.
Why is city MPG lower than highway?
Because every time you brake you convert fuel you already paid for into heat. Stop-and-go traffic does this constantly, while steady highway speed does not — which is why hybrids, which recover braking energy, close much of that gap.